Python 3 compatibility

```
root@xxx:~# bakthat --help
Traceback (most recent call last):
  File "/usr/local/bin/bakthat", line 7, in <module>
    from bakthat import main
  File "/usr/local/lib/python3.5/dist-packages/bakthat/__init__.py", line 175
    except Exception, exc:
                    ^
SyntaxError: invalid syntax```